What this code tells you
The vehicle detected a condition matching the diagnostic criteria for B0021. The code guides testing; it does not confirm which component caused the condition.
Reasonable next steps
- Record all stored and pending codes plus freeze-frame data before clearing anything.
- Check service information for the exact year, make, model and powertrain.
- Inspect related wiring, connectors, hoses and obvious damage before replacing parts.
- Use measured test results or a qualified technician to confirm the root cause.
A flashing check-engine light, severe misfire, overheating, oil-pressure warning, brake warning or unsafe drivability warrants stopping safely and obtaining professional help.